Optical elements such as lenses are usually manufactured by press molding an optical glass by either direct pressing or reheat pressing. Direct pressing is a method according to which temperature of melted glass gob of a predetermined weight is lowered to molding temperature and, at the molding temperature, the glass gob is press molded by a mold.
Reheat pressing is a method according to which glass gob of a predetermined weight obtained by cooling and solidifying melted glass is reheated to the molding temperature region and, at the molding temperature, the glass gob is press molded by a mold.
It is well known that, when glass is held in a devitrification temperature region which is lower than liquid phase temperature of the glass, devitrification occurs and grows, though the speed of growth is low or high depending upon the composition of the optical glass. In a glass having a high liquid phase temperature, the liquid phase temperature widely exceeds the molding temperature region and, therefore, in the above described pressing of the glass, there tends to occur long duration of time during which the glass is held in the devitrification temperature region which is lower than the liquid phase temperature while the temperature of the glass is lowered or raised until molding of the glass and while the glass is being molded and this gives rise to devitrification.
An optical glass having medium refractive index and low dispersion characteristics, particularly an optical glass having optical constants of refractive index (nd) within a range of about 1.54-1.65 and Abbe number (νd) within a range of about 57-69 is useful in optical design. Known in the art as an optical glass having such optical constants are several glasses which are generally known as dense phosphate crown glasses. For increasing refractive index, these glasses mostly comprise ingredients such as Ta2O5 made of expensive raw materials which are added to a phosphate glass which originally has a low refractive index. Since the cost of manufacture of these glasses is high, only a very few of them are manufactured on an industrial scale.
As a glass having similar optical constants to those described above, there is disclosed a P2O5—ZnO optical glass suitable for precision mold pressing (e.g., Patent Literature 1). This glass, however, has the problem that it has a high liquid phase temperature. As described above, a glass having a high liquid phase temperature tends to give rise to devitrification while the temperature of the glass is lowered or raised until molding of the glass and while the glass is being molded and, therefore, it is difficult to manufacture, on a stable basis, optical elements such as lenses which require a high homogeneity. This glass therefore cannot be practically used.
Known also in the art are phosphate optical glasses comprising Ta2O5 (e.g., Patent Literature 2 and 3). The glass containing Ta2O5 however requires a very high cost of raw materials.                Patent Literature 1: Japanese Patent Application Laid-open Publication No. Hei 2-124743        Patent Literature 2 Japanese Patent Publication No. Sho 38-5013        Patent Literature 3 Japanese Patent Application Laid-open Publication No. Sho 52-68217        
It is an object of the present invention to provide an optical glass which has comprehensively eliminated the above described disadvantages of the prior art, has high resistance to devitrification, has medium refractive index and low dispersion characteristics, and is free of expensive raw materials such as Ta2O5 and therefore can be manufactured at a relatively low cost of raw materials.